The present invention is directed generally to optical systems. More particularly, the invention relates to optical wavelength division multiplexing (WDM) systems and optical components employing athermalized optical components, especially Bragg gratings, and methods of making athermalized optical components for use therein.
WDM systems provide an effective means to increase the volume of data transmitted in optical systems. One difficulty that exists with WDM systems is that the various signal wavelengths often have to be separated for routing/switching during transmission and/or reception at the signal destination. In early WDM systems, the wavelength spacing was limited, in part, by the ability to effectively separate wavelengths from the WDM signal at the receiver. Most optical filters in early WDM systems employed a wide pass band filter, which effectively set the minimum spacing of the wavelengths in the WDM system.
Diffraction gratings were proposed for use in many early transmission devices; however, the use of separate optical components in free space configurations were cumbersome and posed problems in early applications. Likewise, etched optical fiber Bragg gratings, while an improvement over diffraction gratings, proved difficult to effectively implement in early operating systems.
Holograpically induced in-fiber Bragg gratings have become well known in the art. See, for instance, U.S. Pat. Nos. 4,725,110 and 4,807,950, which are incorporated herein by reference. Holographically induced Bragg gratings are generally produced exposing an optical waveguide, such a silica-based optical fiber or planar waveguide, to an interference pattern produced by intersecting radiation beams, typically in the ultraviolet frequency range. The intersecting beams can be produced interferometrically using one or more radiation sources or using a phase mask. For examples, see the above references, as well as U.S. Pat. Nos. 5,327,515, 5,351,321, 5,367,588 and 5,745,617, and PCT Publication No. WO 96/36895 and WO 97/21120, which are incorporated herein by reference.
The development of holographically induced in-fiber Bragg gratings has provided a versatile and reliable means to filter closely spaced wavelengths because the wavelength range, or bandwidth, over which the grating is reflective (reflection wavelength) as well as the reflectivity, can be controlled. The filter characteristics of in-fiber Bragg gratings has further improved the viability of dense WDM systems by enabling direct detection of the individually separated wavelengths. For example, see U.S. Pat. No. 5,077,816 issued to Glomb et al. But, one difficulty with Bragg gratings is that the reflection wavelength of the Bragg grating varies as a function of temperature. Specifically, the index of refraction and the length and spacing of the refraction index variations in the grating vary as a function of temperature resulting in a change in the reflection wavelength. In order to achieve high performance WDM systems, reflection wavelength stable Bragg gratings are necessary to allow deployment in a wide range of applications and locations encompassing a wide range of temperature environments.
Athermalization means the process of rendering something as completely independent of temperature or thermal affects. Different methods for athermalization of Bragg gratings have been disclosed. Active temperature control where the Bragg grating is heated and/or cooled has been disclosed, for example, see U.S. Pat. Nos. 6,044,189 and 6,087,280. Active control results in increased cost, weight, and complexity and reduced reliability. Additionally, feedback systems typically have to be included in active control systems to measure the performance of these devices during operation and to control the temperature to achieve the desired reflection wavelength. Again, this adds cost and complexity while reducing reliability. In order to overcome these problems, passive control systems have been developed. These systems use materials with coefficients of thermal expansion to change the strain on the Bragg grating to compensate for the effects of temperature. Various passive systems employ mounting the Bragg grating on a bimetal substrate that bends as a function of temperature resulting in convex and concave sides. Bimetal members are well known in the art. A bimetal member has two layers of metals with different coefficients of thermal expansion bonded together. The difference in the coefficients of thermal expansion causes the bimetal member to bend in one direction or the other depending on the temperature. Therefore, a Bragg grating mounted on the convex side of the bimetal substrate will be subject to a strain that varies as a function of temperature. The bimetal substrate can be designed to apply a strain as a function of temperature that compensates for the variation in the reflection wavelength as a function of temperature resulting in a constant reflection wavelength. Various patents describe passive athermalization of optical devices including U.S. Pat. Nos. 5,841,920, 5,844,667, 6,044,189, 6,087,280, 6,101,301, and 6,108,470.
While mounting a Bragg grating on a bimetal substrate can provide passive control of the reflection wavelength, temperature variations can fatigue the system, which decreases the reliability and lifetime of the system. For example, current techniques for attaching the Bragg grating to the substrate slip over time leading to improper strain on the fiber. Current attachment techniques can also lead to fiber breakage because of the movement due to temperature changes. For example, a fiber attached with a hard bond such as a hard epoxy may crack and eventually slip unless any difference in the coefficient of thermal expansion of the fiber 101 or the temperature compensating substrate 104 is accommodated for in the design. A hard attachment can also cause the fiber 101 to break, because as the fiber moves with the temperature compensating substrate 104 as the temperature changes, the fiber can be subject to sharp bending at the hard attachment point. Repeated temperature cycles can cause repeated sharp bending of the fiber at the attachment point resulting in the fiber breaking. On the other hand, if the fiber is attached with a soft bond, such as a soft epoxy, the fiber 101 can slip or move enough to affect the reflection wavelength of the Bragg grating 102. In addition, current passive athermalization techniques do not adequately compensate for non-linear variations in Bragg grating reflection wavelength due to temperature variations. There remains a need for more wavelength stable and more reliable athermalized components and especially Bragg gratings. Also there remains a need for a method of attaching a fiber containing a Bragg grating to a temperature compensating substrate that will not crack or slip.
Other passive systems for passive athermalization control involve the use of linear package designs, in which, materials with different coefficients of thermal expansion a connected in parallel to a fiber to provide temperature compensation. In practice the length and compliance of the fiber anchoring points and the joints between the high and low coefficient of thermal expansion materials make it difficult to know and control the exact effective lengths of the package components. Manufacturing difficulties have limited the precision of compensation, yield, and cost of this style of device. For example, accuracy of attachment of the fiber to the package and accuracy of the relative effective lengths of the package materials affects the precision of the compensation for temperature. There remains a need for linear package designs that can be more easily manufactured and yet provide precision compensation.
Accordingly, the present invention addresses the aforementioned desires to provide athermalized components that have increased wavelength stability and reliability. These advantages and others will become apparent from the following detailed description.